decoder: make decode_step() return once the frame is decoded.
decoder: make decode_step() return once the frame is decoded.

Make sure we always have a free surface left to use for decoding the
current frame. This means that decode_step() has to return once a frame
gets decoded. If the current adapter contains more buffers with valid
frames, they will get parsed and decoded on subsequent iterations.

GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi